Transmissions that have constraints on the size of the diameter of gears due to space limitations, but which are also required to transmit significant power, have up to now presented difficult design problems. An appealing concept for the solution to this problem is the use of multiple gears on a common shaft. Each set of engaging gears on a common drive shaft and a common driven shaft would transmit a relatively low load. However, the combined load transmitted by a plurality of sets of engaging gears on a common drive shaft and a common driven shaft would be relatively large. This apparent simple theoretical solution has proven to be impractical because of the difficulty in achieving even load sharing among sets gears.
Due to gear manufacturing tolerances, multiple gears on a drive shaft are not likely to be perfectly aligned with the corresponding multiple gears on a driven shaft. When load is applied, sets of gears on the drive and driven shafts are not likely to simultaneously mate. This non-perfect alignment results in sets of gears on the drive and driven shaft to engage unevenly. This results in one or more sets of gears to be loaded higher than their design load with resulting premature failure due to excessive load or wear. Minor misalignments in the drive and driven shafts and shaft flex also are factors contributing to the uneven loading of engaging sets of gears on the drive and driven shafts.
Up until the time of the present invention, very precise tolerances were required to successfully construct a transmission having multiple gears on a common shaft. As is appreciated by one skilled in the art, the more precise the tolerances required, the greater are the manufacturing costs. Because of these tolerance-cost considerations, transmissions having multiple gears on a single shaft were generally impractical.